Regiospecific process for synthesis of acyclic nucleosides

ABSTRACT

This invention relates to an improved regiospecific process for the synthesis of acyclic nucleosides such as, acyclovir and ganciclovir, anti-viral compounds especially effective against herpes virus, and intermediates thereof starting from dyacylguanine and an alkylating agent, selected from 2-oxa-1,4-butanediol diacetate (OBDDA), 1,4-diacetoxy-3-acetoxymethyl-2-oxa-butane, 1,4-dibenzyloxy-3-acetoxymethyl-2-oxabutane. The reaction is carried out in the absence of an acid catalyst and a solvent.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved regiospecific process for thesynthesis of acyclic nucleosides such as, acyclovir and ganciclovir,anti-viral compounds especially effective against herpes virus, andintermediates thereof starting from diacylguanine and an alkylatingagent, selected from 2-oxa-1,4-butanediol diacetate (OBDDA),1,4-diacetoxy-3-acetoxymethyl-2-oxa-butane,1,4-dibenzyloxy-3-acetoxymethyl-2-oxabutane.

2. Discussions of the Background

Both acyclovir (Ia) and ganciclovir (Ib) show remarkable anti-viralactivities (U.S. Pat. No. 4,199,574, U.S. Pat. No. 4,355,032) ##STR1##

    Ia:R=--CH.sub.2 --O--CH.sub.2 CH.sub.2 --OH (Acyclovir)

    Ib:R=CH.sub.2 --O--CH(CH.sub.2 OH)CH.sub.2 OH (Ganciclovir)

The strategy adopted in prior art for manufacture of I is alkylation ofappropriately substituted 2-aminopurines e.g. guanine derivatives withrequired addendum which essentially includes an acid with or withoutsolvent to yield N-9 alkylated intermediate (II) e.g. N² -acetyl-9-(2-acetoxy ethoxy)methyl! guanine (IIa) along with corresponding N-7alkylated isomer (IIIa). In particular, it is known to react mono ordiacetylated guanine with 2-oxa-1,4-butanediol diacetate (OBDDA) toyield the intermediate compound of formula II. ##STR2##

    IIa:R.sup.1 =H, R.sup.2 =COCH.sub.3

    IIb:R.sup.1 =CH.sub.2 OCH.sub.2 Ph, R.sup.2 =CH.sub.2 Ph

    IIc:R.sup.1 =CH.sub.2 OCOCH.sub.3, R.sup.2 =COCH.sub.3

along with compound of formula III ##STR3##

Wherein the formula III includes the compound according to IIIa, IIIband IIIc, with R¹ and R² as defined in formula IIa, IIb and IIcrespectively.

This intermediate mixture of II and III is purified generally by costlyand tedious processes to remove the last traces of N-7 isomer (III) andthen hydrolysed to yield I. Often N-7 isomer (IV) is associated withfinal product which is further removed employing a number of operations.##STR4##

Wherein for IVa and IVb, R is as defined in formula I.

Since alkylation of guanine derivatives like diacetyl/monoacetyl guanine(DAG/MAG) of formula V and VI ##STR5## (Va:R⁴ =COCH₃, DAG) ##STR6##(VIa:R³ =COCH₃, MAG) wherein R³ =--CHO,

where R⁵ =methyl, ethyl, isopropyl, phenyl;

in presence of an acid, is a thermodynamically controlled reaction, N-7isomer of formula III is always formed. However, N-9 isomer (II) beingthermodynamically more stable (Chem Pharm Bull, 1970, 18, 1446) isproduced as the major product.

The formation of N-7 isomer (III) increases the total cost ofmanufacture. Hence, there is a need for development of regiospecificprocess for the manufacture of II, the penultimate intermediate for I.

Some of the important methods for manufacture of acyclovir/ganciclovirof formula I reported in the prior art are described below:

1. BE 833 006, U.S. Pat. No. 4,199,547 describe a process for thesynthesis of Acyclovir (Ia) which involves condensation oftrimethylsilylated guanine with 2-benzoyloxyethyoxy methyl chloride inDMF in the presence of a base followed by deprotection, yielding in thedesired N-9 isomer(IIa) along with unacceptable amounts of itscorresponding N-7 isomer(IIIa). The former after purification anddeacetylation gives acyclovir in 24% overall yield.

2. Matsumoto in Chem Pharm Bull, 36(3), 1153-1157 and JP 63-107982teaches a process for the synthesis of acyclovir(Ia) by condensation ofdiacetyl guanine (DAG Va) with 2-oxa-1,4-butanediol diacetate (OBDDA,VIIa) in DMSO in presence of an acid catalyst to get a mixture ofN-9/N-7 isomer (66:26). The former is isolated by column chromatographyand deacetylated with methanolic ammonia to give acyclovir. Overallyield of acyclovir from guanine is 42%

The process is illustrated in the following scheme A: ##STR7## 3. JP59-80685 utilises similar chemistry but starts from N² -monoacetylguanine (MAG, VIb) in presence or absence of solvent to yield a mixtureof N-9 and N-7 alkylated guanine derivatives (N-9/N-7 ratio 52:26). Theformer subsequently isolated and deprotected to give acyclovir inoverall 43% yield.

4. EP 532 878 describes a process in which a transgly cosilationreaction between guanosine, acetic anhydride and 2-oxa-1,4-butanedioldiacetate (OBDDA) in the presence of catalytic amount of an acid iscarried out, followed by hydrolysis to yield a mixture of acyclovir (Ia)and its N-7 isomer (IVa). The above process is illustrated in thefollowing scheme B: ##STR8##

It can be observed that the major drawback in all the processesdescribed above is that acyclovir (Ia) or its Intermediate (IIa) isalways contaminated with substantial amount of its N-7 isomer, and hencethe separation of the desired N-9 isomer from the mixture is verytedious and requires chromatographic separation or fractionalcrystallisation.

In addition to the above, the PCT patent specification WO 95/07281describes a process for the synthesis of Acyclovir (Ia) from N²-formylguanine. The chemistry of the process is illustrated in thefollowing Scheme C: ##STR9##

As illustrated in the above scheme, acyclovir is synthesised in foursteps:

1) condensation reaction between guanine and glyoxal,

2) oxidation of the resulting tricyclic vicinal diol to N² -formylguanine

3) alkylation of N² -formylguanine with OBDDA in the presence of anacid,

4) hydrolysis of N² -formyl-N⁹ alkylated guanine to Acyclovir.

The final crude produce is subjected to elaborate purification steps toget guanine free acyclovir of pharmaceutical grade. It is a lengthyprocess and hence not practical.

It might be highlighted that in the prior art, alkylation of DAG (Va),N² -acetyl guanine(MAG, Via) or N² -formylguanine to the penultimateintermediate of I is always an acid catalysed one; for instancep-toluenesulfonic acid is a common acid although other acids have alsobeen employed Chem Pharm Bull, 36(3), 1153-1157 (1988)!. Also use of asolvent is preferred in almost all the cases and no attempt has beenmade for recycle of the undesired N-7 isomer or developing conditionswhich can lead to selective formation of the desired isomer i.e. II.

Prior art also does not specify the molar ratio of acid catalyst andalkylating agent with respect to protected guanine derivatives forobtaining high N-9/N-7 isomer ratio.

SUMMARY OF THE INVENTION

Accordingly, the basic objective of the present invention is to providefor a simple and cost effective regiospecific process for the synthesisof N-9 alkylated guanine derivatives of general formula II such as N²-acetyl-9- 2-(acetoxyethoxy)methyl!guanine(IIa), N² -acetyl-9-1,3-bis(benzyloxy-2-propoxy)methyl!guanine(IIb), which would be suitableintermediates for the manufacture of acyclic nucleosides of formula Isuch as acyclovir and ganciclovir having desired characteristics for itstherapeutic use as anti-viral agents.

A further object of the invention is to provide a process for therecycle of the N-7 isomer of formula III to its thermodynamically morestable N-9 isomer of formula II.

Other objects will be apparent from the description of the inventiongiven herein below.

In accordance with the above basic objectives of the present inventionin an aspect of the invention there is provided a regiospecific processfor the manufacture of intermediate compound of formula I for use insynthesis of acyclic nucleocides of formula I which comprises reacting asubstituted guanine derivative of formula V, ##STR10## wherein ##STR11##where R⁵ =methyl, ethyl, isopropyl, phenyl with alkylating agent offormula VII,

wherein R¹ and R² is as defined in formula II, without the presence ofany acid catalyst and/or solvent under modified conditions comprisingcarrying out said reaction between protected guanine derivative and saidalkalyting agent in the molar ratio of 1.5 to 6.0 preferably 1.5 to 2.5at a temperature ranging from 90°-170° preferably between 100° C.-110°C. for a period of 75-80 hrs.

Such an aspect of the invention is schematically represented as follows:##STR12## Wherein formula VII is representative of formuli VIIa, VIIband VIIc, with R¹ and R² as defined in formula IIa, IIb and IIcrespectively.

Another aspect of the invention there is provided a process forproducing practically pure acyclic nucleocides of formula I such asacyclovir and ganciclovir from the compound of formula II obtained asabove by basic hydrolysis. Such an aspect of the invention isrepresented by the following steps:

i) washing the crude intermediate (II) with a solvent selected frommethanol, ethanol, iso-propanol, acetone, THF, dioxane, dimethoxyethane,acetonitrile, toluene, benzene, ethyl acetate, dichloromethane ormixture thereof to remove the traces of non-polar impurities.

ii) deprotection of the various functional groups such as esters, orbenzyl ethers of the intermediate (II) to yield pure N-9 isomer (I) i.eacyclovir or ganciclovir of extremely high purity.

Another aspect of the invention provides for a simple method ofrecycling N-7 isomer of formula III to its more thermodynamically stableN-9 isomer of formula II by heating said N-7 isomer in the presence ofOBDDA without any acid or solvent in the temperature range of 100°-110°C. for a period of 10-20 hours preferably 13-15 hours to thereby producea mix of N-9and N-7 isomer the former constituting the major portion ofsaid mix.

Other aspects of the invention will become apparent from the descriptionof the present invention given herein below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the relationship between percentage ofN-9 isomer and concentration of p-TsOH.

FIG. 2 is a graph illustrating the relationship between percentage ofN-9 isomer and reaction time for three different concentrations ofp-TsOH.

FIG. 3 is a graph illustrating the relationship between the percentagesof N-7 and N-9 isomer and the relative amounts of OBDDA and DAG.

FIGS. 4A and 4B illustrate the relationship between percentage of N-7and N-9 isomer and the reaction time.

DETAILED DESCRIPTION OF THE INVENTION

With the objectives of the present invention discussed above in mind,extensive study on the subject made by the present inventors revealedthat in the presence of an acid catalyst and a solvent, the alkylationreaction of DAG(Va) and OBDDA, gives significant proportion of N-7isomer (III) along with other side products and also poor yields of N-9isomer (II). Thus, in a homogeneous mixture of DAG(Va), OBDDA, p-toluenesulfonic acid (p-TsOH) and a solvent, the ratio of N-9 to N-7 isomervaries from 2.3 to 2.6. Further, with higher concentration of acid theratio of N-9/N-7 isomer is observed to come down substantially as isclearly illustrated in Table 1 hereunder.

                  TABLE 1                                                         ______________________________________                                        Effect of various concentrations of p-TsOH on N-9/N-7 ratio                   (of II & III)                                                                 Reactants:                                                                    Diacetyl guanine (Va):                                                                      10 gms (0.0425 moles)                                           OBDDA (VIIa): 18.7 gms (0.106 moles)                                          Reaction Conditions:                                                                        Reaction mixture heated at                                                    100° C.-105° C. in a round bottom flask.                                  HPLC monitoring of                                    Amt of                  condensation reaction                                 p-TsOH.H.sub.2 O used         % ratio                                                  molar React-               calculated                                                   ratio ion    N-9  N-7  N-9  N-7                            Sr         moles ×                                                                         pTsOH/                                                                              Time   iso- iso- iso- iso-                           No  Gms    10.sup.-3                                                                             DAG   Hr     mer  mer  mer  mer                            ______________________________________                                        1   0      0       0     70     95.60                                                                              2.90 97.05                                                                              2.94                           2   0.05    0.265  0.0063                                                                              28     92.60                                                                              3.70 96.45                                                                              3.54                           3   0.10   0.53    0.0125                                                                              21     90.10                                                                              5.10 94.64                                                                              5.36                           4   0.19   1.06    0.0250                                                                              15     82.90                                                                              5.70 93.56                                                                              6.43                           5   0.40   2.12    0.050 15     80.20                                                                              6.50 92.50                                                                              7.50                           6   0.80   4.21    0.100 15     77.50                                                                              7.90 90.70                                                                              9.20                           ______________________________________                                    

From Table 1, one would observe that with higher concentration of acid,the ratio of N-9/N-7 isomer comes down substantially. The same data isplotted in FIG. 1.

The inventors further observed that N² -acetylguanine i.e. N²-monoacetylguanine(MAG, VIa) cannot be alkylated in the absence of acidcatalyst by OBDDA. However, surprisingly enough the inventors haveidentified that the compound of formula V such as diacetylguanine (DAG,Va) can be converted to the intermediate (II) in the presence of OBDDAor the corresponding alkylating agents used to synthesis ganciclovirwithout the aid of any acid catalyst or solvent only when reacted underspecific modified reaction conditions.

Thus, the present inventors have found that under said specific modifiedreaction conditions and in the absence of any acid catalyst and solventalthough the reaction between DAG (Va) and OBDDA was slower but suchconditions provided much purer N-9 isomer (IIa) with very littleconcomitant formation of the undesired N-7 isomer. Such findings of theinvention are illustrated hereunder in Table 2 which indicates the rateof formation of N-9 isomer of formula IIa using two differentconcentrations of the acid catalyst viz. p-toluene sulphonic acid andwithout any such acid. The formation of N-9 isomer was monitered throughHPLC analysis and the results are further plotted in FIG. 2.

                  TABLE 2                                                         ______________________________________                                        Comparison of rate of formation of N-9 isomer in the presence                 and absence of p-toluene sulfonic acid.H.sub.2 O (p-TsOH.H.sub.2 O).          Reactants:                                                                    Diacetyl guanine (Va):                                                        10 gms (0.0425 moles)                                                         OBDDA (VIIa):                                                                 18.7 gms (0.106 moles)                                                        Reaction Conditions:                                                                          Reaction mixture heated at                                                    100°C.-105° C.,                                 P-TsOH.H.sub.2 O                                                              moles × 10.sup.-3                                                                     A         B      C                                              Sr       Reaction 4.21      1.06 0                                            No.      Times Hrs                                                                              % N-9 Isomer formed*                                        ______________________________________                                        1        03       55.20     28.50                                                                              15.1                                         2        06       69.10     49.20                                                                              24.6                                         3        09       70.20     69.10                                                                              31.9                                         4        12       73.10     77.20                                                                              44.16                                        5        15       76.30     80.50                                                                              85.3 (44 hrs)                                6        21       77.50     81.20                                                                              95.6 (70 hrs)                                ______________________________________                                         *Based on HPLC monitoring of the alkylation step.                        

The above table 2 clearly reveals that the formation of N-9 isomer ismore when the alkylation reaction is carried out in the absence of anyacid as compared to the use of the acid when the reaction time isallowed to proceed beyond 15 hrs.

With such findings of favourable yield of N-9 isomer in the absence ofany acid catalyst for the alkylating reaction of diacetyl guanine withOBDDA, the dependants of molar proportion of the two reactants on theyield of N-9 isomer of formula II in the absence of acid was determined.The results are reproduced hereunder under Table 3 and furtherrepresented in FIG. 3.

                  TABLE 3                                                         ______________________________________                                        Effect of concentration of OBDDA on N-9/N-7 isomer formation                  without acid catalyst                                                         Reaction temperature:                                                                           100° C.-105° C.                               Sr  OBDDA Used  % N-9 isomer*                                                                            % N-7 isomer*                                                                          Reaction time                             No  moles/mole DAG                                                                            IIa        IIIa     Hrs                                       ______________________________________                                        1   2.0         95.6       2.9      75                                        2   3.0         93.0       3.6      60                                        3   4.0         91.5       4.5      46                                        4   5.0         90.6       4.7      38                                        5   6.0         89.0       5.4      33                                        ______________________________________                                         *determined by HPLC monitoring of the reaction.                          

As evident from table 3 above, though with higher proportion of OBDDAthe rate of reaction is faster, however yields tend to be lower and thebest yield is obtained when approximately two moles of OBDDA per mole ofDAG(Va) is used. Thus, when the molar ratio between OBDDA and DAG is 2and no acid or no solvent is used and the mixture is heated at 100°C.-110° C. for 75-80 hour, a mixture of >95% of N-9 isomer of IIa and<3% of the corresponding N-7 isomer is obtained.

The observation that the alkylation of DAG(Va) with OBDDA,1,4-diacetoxy-3-acetoxymethyl-2-oxa-butane or similar alkylating agents,is also a thermodynamically controlled reaction even in the absence ofan acid catalyst and a solvent is novel.

Thus the above teachings further indicated that either N-9 isomer(II) orits corresponding N-7 isomer (III) could be equilibrated to a mixture ofboth the isomers even in the absence of acid and solvent.

The invention accordingly thus further identifies that when theaforesaid N-9 isomer or N-7 isomer is heated at 100° C.-110° C. in thepresence of OBDDA and in the absence of an acid and solvent, a mixtureof N-9 and N-7 isomers is indeed obtained, the former being the majorproduct.

The rate of isomerisation of N-7 isomer to N-9 isomer in the presenceand absence of an acid was studied and the details are representedhereunder in Table 4 which is further illustrated in FIG. 4a and 4b.

                  TABLE 4                                                         ______________________________________                                        Equilibrium study of N-7 isomer (IIIa) at 100° C.-104° C.            Reaction                                                                 Sr   Time        With p-TsOH     Without P-TsOH                               No   Hrs         % N-9   % N-7   % N-9 % N-7                                  ______________________________________                                        1    00.083      09.57   81.13   --    --                                     2    00.330      45.00   50.40   --    --                                     3    00.660      62.43   32.88   --    --                                     4    01.000      76.90   19.58   24.67 66.55                                  5    02.000      89.50   07.62   33.93 61.76                                  6    03.000      91.30   06.88   46.84 46.49                                  7    04.000      90.63   05.90   62.38 33.37                                  8    06.000      92.60   05.43   83.40 12.98                                  9    09.000      91.00   05.54   91.84 03.89                                  10   12.000      90.84   05.36   94.50 03.33                                  11   15.000      91.44   04.49   95.70 02.72                                  ______________________________________                                    

The above results in table 4 therefore clearly illustrate that in thepresence of an acid catalyst N-7 isomer isomerises to 91.44% of N-9isomer with 4.5% of N-7 isomer remaining, whereas in the absence of acidunder identical conditions the isomerisation takes place to the extentof 95% of N-9 isomer in 15 hours. The isomerisation of pure N-9isomer(IIa) when studied under identical conditions, was found to givealmost similar ratios of N-9 and N-7 isomers in acid catalysed andnon-catalysed reactions as reported (see table IV) for N-7 isomer.

The effect of temperature on the rate of reaction on the ratio of isomerdistribution and the yield is quite significant. It has been observedthat the yield of N-7 isomer is more at higher temperatures than that atlower temperatures.

Moreover, an acid catalysed reaction at higher temperatures gives loweryields than that of the reaction without any acid catalyst. Highertemperature tends to produce more impurities/side products.

Thus by way of the above the present invention provides that undermodified conditions comprising fusion reaction between protected guaninederivatives of structure (V) and alkylating agent (VII) in the molarratio of 1.5 to 6 at a temperature ranging from 90°-170° C., preferablybetween 100° C.-110° C., in absence of solvent of catalyst for 75-80hours it is possible to obtain the compound of formula II and alsosynthesis of acyclic nucleosides from such compounds of formula II byway of a simple and cost effective process of manufacture.

The objects, advantageous and means of attaining the same as also thescope of the present invention will hereinafter be illustrated ingreater detail by way of the following non-limiting examples. It shouldbe understood that the invention is not intended to be limited to thespecific examples.

EXAMPLES Example 1

N², N⁹ Diacetyl guanine(DAG, Va):

A mechanically stirred heterogeneous mixture of guanine (22.65 g, 0.15mole), acetic anhydride (91.8 g, 0.9 mole) and p-toluene sulphonic acidmonohydrate (0.70 g, 3.75×10⁻³ mole) is heated at 130° C.-135° C. for12-14 hours. After the completion of the reaction (monitored by HPLC),the reaction mixture is cooled to room temperature and filtered usingcintered funnel. The solid thus obtained is washed with toluene (3×25ml) and dried at 70° C.-80° C. under vacuum for 4-5 hours to give 33.1 g(93.9%) diacetyl guanine (DAG), m.p. 300° C.(dec)

PMR(DMSO-d₆)δppm: 8.5(s, 1H, H), 2.85 (s,3H, N⁹ --COCH₃),2.2(s,3H, N²--COCH₃)

IR(KBr)cm⁻¹ : 3150, 1720, 1705, 1685, 1605, 1528, 1220, 619.

HPLC Conditions: Column C₁₈ (reverse phase); eluent, CH₃ CN: water;20:80 adjusted to pH2; detector, UV 254 nm, flow rate 2 ml per minute.

Example 2

N² -Acetyl-9- (2-acetoxyethoxy)methyl! guanine (N-9 isomer, IIa) withacid catalyst.

A stirred mixture of DAG (10 g, 0.0425 mole), OBDDA(18.7 g; 0.106 mole)and p-TsOH.H₂ O (0.19 g, 0.001 mole) was heated in a round bottom flaskat 105° C.-110° C. for 15 hours. The reaction mixture was concentratedunder vacuum and the residue was column chromatographed on SiO₂ columnusing CH₂ Cl₂ --CH₃ OH (60:40 v/v) solvent system to give the desiredN-9 isomer in 85-86% isolated yields, m.p. 189° C.-190° C.

PMR(DMSO-d₆ (δ ppm: 12.1 (bs, 1H, HNCOCH₃) 11.85 (bs, 1H, NH), 8.2 (s,1H, H), 5.5 (s, 2H, NCH₂ O), 4.15 (m, 2H, OCH₂), 3.7 (m, 2H, OCH₂), 2.2(s, 3H, NCOCH₃), 1.95 (s, 3H, NCOCH₃).

Example 3

N² -Acetyl-9- (2-acetoxyethoxy)methyl! guanine (N-9 isomer IIa) withoutacid catalyst

Diacetylguanine (DAG) (10 g, 0.0425 mole) and OBDDA (18.7 g, 0.106 mole)is heated in a round bottom flask at 105° C.-110° C. under continuousstirring for 75-80 hours. After almost complete conversion of DAG, theexcess of OBDDA was distilled out and the residue was heated with amixture of toluene:methanol (25:75, 25 ml) at 45°-50° C. for 30 minutesand the solid was collected by filtration at 5° C.-10° C. to yield 98.8%pure (by HPLC) N-9 isomer in 91.0% isolated yield.

Example 4

N² -Acetyl-9- (2-acetoxyethoxy)methyl! guanine (N-9 isomer, IIa):

A stirred mixture of DAG (10 g, 0.042 mole) and OBDDA (18.7 g; 0.106mole) was heated in a round bottom flask at 105° C.-110° C. for 80hours. The reaction mixture was concentrated under vacuum and theresidue was column chromatographed on SiO₂ column using CH₂ Cl₂ :MeOH(6:4 v/v) to give the desired N-9 isomer in ≧94% isolated yields, m.p.189° C.-190° C.

Example 5

N² -Acetyl-9- (2-acetoxyethoxy)methyl! guanine (N-9 isomer, IIa):

A stirred mixture of DAG (10 g, 0.0425 mole), OBDDA (44.9 g; 0.255 mole)and p-TsOH.H₂ O (0.19 g, 0.001 mole) was heated in a round bottom flaskat 100° C.-105° C. for 20 hours. Excess OBDDA was removed under vacuumand the residue thus obtained was diluted with toluene (50 ml) andheated at 100° C. for 2 hours. The reaction mixture is then cooled at50° C. and filtered to get 11.30 g (87%) crude product.

Purification of crude product

A suspension of the above obtained crude N-9 isomer (11.30 g) intoluene:isopropyl alcohol (1:1, 50 ml) was heated in round bottom flaskat 75° C.-80° C. for 2 hours. The reaction mixture is then cooled to 0°C.-5° C., maintained at the same temperature for 1 hour and filtered toget 10.6 g (80%) N-9 isomer of more than 99% HPLC purity.

Example 6

N² -Acetyl-7- (2-acetoxyethoxy)methyl! guanine (N-7 isomer):

A mixture of DAG (23.5 g, 0.1 mole), OBDDA (61.6 g; 0.35 mole) andp-TsOH.H₂ O (0.475 g, 2.5×10⁻³ mole) in acetic acid (75 ml) was heatedin a round bottom flask at about 110° C. under continuous stirring for 8hours. The temperature of the reaction mixture was brought down to 50°C. and acetic acid was removed by distillation under vacuum. The residueobtained after removing acetic acid was extracted with benzene (3×50 ml)and the combined extract was concentrated under vacuum to give a thickoily residue which was purified by passing through a column of SiO₂using CH₂ Cl₂ --CH₃ OH(80:20 v/v) solvent system to yield N-7 isomer (10g) of 87% HPLC purity.

PMR (DMSO-d₆) ppm: 12.5 (bs, 1H, CONH); 11.6 (bs, 1H, CONH); 8.2 (s, 1H,H), 5.5 (s, 2H N--CH₂ --O); 4.1 (m, 2H, OCH₂ O); 3.70 (m, 2H, OCH₂); 2.2(s, 3H, OCOCH₃) 1.95 (s, 3H, NCOCH₃).

Example 7

Isomerisation of N-7 isomer to N-9 isomer in presence of p-TsOH

A heterogeneous mixture of N-7 isomer (0.4 g, 0.013 mole; 87% pure) andOBDDA (1.14 g, 0.065 mole) in presence of p-TsOH.H₂ O (0.062 g,0.325×10⁻³ mole) was heated at 100° C.-110° C. under continuous stirringand the progress of the reaction was monitored by HPLC. Samples werewithdrawn at regular time intervals and analysed by HPLC. The data ispresented in Table IV.

Example 8

Isomerisation of N-7 to N-9 in absence of p-TsOH

The isomerisation of the N-7 isomer in absence of P-TsOH was performedunder identical conditions as given in the above experiment (e.g. 7).Samples were withdrawn at regular time intervals and analysed by HPLCtechniques. The data is presented in Table IV. The transformation of N-7to N-9 isomer as monitored by HPLC was found to be comparatively veryslow and took almost 15 hours to reach equilibrium (for details seeTable-IV, FIG. 4b).

Example 9

N² -acetyl-9 (2-acetoxyethoxy)methyl! guanine: Effect of concentrationof acid catalyst on the rate of the reaction

A suspension of DAG (10 g, 0.0425 moles) in OBDDA (18.7 g, 0.106 moles)was heated in the presence of different amounts of TsOH.H₂ O and also inthe absence of the latter at 100° C.-105° C. Samples were withdrawn atregular time intervals and analysed by HPLC. The data is presented inTable II.

Example 10

9- (2-Hydroxyethoxy)methyl! guanine (Acyclovir):

To a solution of NaOH pellets (3.8 g, 0.097 mole) in water (100 ml) isadded N-9 isomer (10 g, 0.323 mole) at room temperature. The reactionmixture is heated at 85° C.-95° C. for 3 hours. After bringing thetemperature down to room temperature the pH of the clear solution isadjusted to pH 7 using 35% HCl and filtered to yield ≧95% of the productof very high quality, m.p. 253° C.

PMR(DMSO-d₆)δ ppm: 10.7 (s, 1H, NH), 7.85 (s,1H, H), 6.51 (s, 2H, N--CH₂--O) 5.3 (s, 2H, NH₂); 4.70 (m, 1H, OH), 3.45 (m, 2H, OCH₂), 3.3 (m, 2H,OCH₂).

Example 11

N² -Acetyl-9- (1,3-bis(benzyloxy)-2-propxy) methyl! guanine (N-9 isomer;II_(b)):

A mixture of DAG (5 g, 0.021 mole),2-O-(acetoxymethyl)-1,3-di-O-benzylglycerol (10.9 g, 0.032 mole) washeated in a round bottom flask under stirring at 110° C.-115° C. for75-80 hours. The reaction mixture was cooled to room temperature andextracted with hexane (3×15 ml). The residue thus obtained was columnchromatographed on SiO₂ column using ethylacetate:hexane (50:50 v/v)solvent system to give the desired N-9 isomer (m.p 147° C.) in 68.7%yield. The yield of the N-7 isomer (m.p 133° C.-134° C.) obtained fromcolumn was found to be 14.86%.

PMR (DMSO-d₆ ppm (N-9 isomer): 8.13 (s, H, H-8), 7.35 (m, 10H, ArH),5.59 (s, 2H, H-1'); 4.41 (s, 4H, benzylic), 4.05 (m, 1H, H-4'), 3.41 (m,4H, H-3' & H-5'); 2.18(s, 3H, CH₃)

PMR (DMSO d₆) ppm (N-7 isomer): 8.34 (s, H, H-8), 7.35 (m, 10H, ArH),5.80 (s, 2H, H-1'); 4.42 (s, 4H, benzylic), 4.14 (m, 1H, H-4'), 3.48 (m,4H, H-3' & H-5'); 2.19(s, 3H, CH₃)

Example 12

9- (1,3-dihydroxy-2-propoxy)methyl!guanine (ganciclovir):

The synthesis of Ganciclovir starting from N² -acetyl-9 1,3-bis(benzyloxy)-2-propoxy) methyl! guanine (N-9 isomer, II_(b)) obtainedfrom example. No. 11 was carried out following the reported conditions(J C Martin et al J. Med. Chem. 1983, 26, 759-761) to get the desiredproduct of high purity in 76% isolated yield, m.p.>300° C.

PMR (DMSO-d₆)δ: 10.64(bs, 1H, NH), 7.81(s 1H, H-8), 6.5 (s, 2H, NH₂),5.44(s, 2H, H-1'), 4.63 (p, J=6 Hz, 1H, H-4'), 3.35 (m, H-3' & H-5')

We claim:
 1. A regiospecific process for the synthesis of compounds offormula II, ##STR13## wherein IIa:R¹ =H, R² =COCH₃ IIb:R¹ =CH₂ OCH₂ Ph,R² =CH₂ Ph IIc:R¹ =CH₂ OCOCH₃, R² =COCH₃ which comprises reacting asubstituted guanine derivative of formula V, ##STR14## wherein ##STR15##where R⁵ =methyl, ethyl, isopropyl, phenyl with alkylating agent offormula VII, ##STR16## wherein R¹ and R² are as defined in formula II,without the presence of any acid catalyst and/or solvent under modifiedconditions comprising carrying out said reaction between the substitutedguanine derivative and said alkalyting agent in the molar ratio of 1.5to 6.0 at a temperature ranging from 90°-170° for a period of 75-80 hrs.2. A process as claimed in claim 1 wherein the compound of formula II isN² -acetyl-9- (2-acetoxyethoxy)methyl!guanine (IIa) obtained by reactingdiacetylguanine (DAG) of formula V with 2-oxa-1,4-butandiol diacetate(OBDDA) of formula VII.
 3. A process as claimed in claim 1 wherein thecompound of formula II is N² -acetyl-9-1,3-bis(benzyloxy)-2-propoxy)methyl!guanine(IIb) obtained by reactingdiacetyl-guanine (DAG, Va) with1,4-dibenzyloxy-3-acetoxymethyl-2-oxabutane of formula VII.
 4. A processof claim 1 wherein the compound of formula II is purified by washingwith solvents selected from methanol, ethanol, iso-propanol, dioxane,THF, 1,2-dimethyoxyethane, acetonitrile, toluene, benzene,dichloromethane, ethyl acetate or mixture thereof.
 5. A process asclaimed in claim 1, wherein the molar ratio is from 1.5 to 2.5.
 6. Aprocess as claimed in claim 1, wherein the temperature range is from100° C.-110° C.
 7. A process as claimed in claim 5, wherein thetemperature range is from 100° C.-110° C.